1. Field of the Invention
The present invention relates to a vacuum valve controller for opening and closing a vacuum valve mounted on an end of a pipe of a vacuum system that is part of a vacuum sewage system.
2. Description of the Related Art
Heretofore, there has been known a vacuum sewage system which includes a vacuum system having a pipe connected to a sewage pit and delivers sewage stored in the sewage pit to a predetermined site such as a sewage treatment plant under a vacuum pressure developed in the pipe of the vacuum system. The vacuum sewage system has a sewage suction pipe placed in the sewage pit, a vacuum valve for selectively connecting the sewage suction pipe to and disconnecting the sewage suction pipe from the pipe of the vacuum system, and a vacuum valve controller for opening and closing the vacuum valve depending on the water level of sewage in the sewage pit.
FIG. 1 of the accompanying drawings is a sectional side view showing a sewage pit 300 and its associated components used in the conventional vacuum sewage system. As shown in FIG. 1, the sewage pit 300, which is placed in the ground, has a sewage tank 301 for holding sewage therein, a suction pipe 303 having a tip end disposed in the sewage tank 301, a vacuum valve 307 provided between the other end of the suction pipe 303 and a vacuum sewage pipe 305 of a vacuum system, a pressure sensor (pressure sensor pipe) 309 for converting a water level change in the sewage tank 301 into a pressure change, and a vacuum valve controller 311 for opening and closing the vacuum valve 307 depending on the pressure change detected by the pressure sensor 309. For example, the vacuum valve controller is disclosed in Japanese laid-open patent publication No. H2-289730. Further, a gravity flow-type sewage inlet pipe 313, and a breather pipe 315 for introducing the atmospheric pressure used by the vacuum valve controller 311 from the location above the ground which is not submerged are connected to the sewage pit 300.
When sewage flows from the sewage inlet pipe 313 into the sewage pit 300 and a certain amount of sewage is stored in the sewage tank 301, the pressure of the air in the pressure sensor 309 increases, and the pressure of the pressure sensor 309 is transmitted to the vacuum valve controller 311. When the pressure rise in the pressure sensor 309 reaches a predetermined level, the vacuum valve controller 311 supplies a negative pressure introduced from the vacuum sewage pipe 305 to the vacuum valve 307, thereby opening the vacuum valve 307, drawing the sewage in the sewage tank 301 through the suction pipe 303 into the vacuum sewage pipe 305, and draining the sewage. When the amount of sewage in the sewage tank 301 decreases as it is drained, the pressure in the pressure sensor 309 is lowered. When the pressure in the pressure sensor 309 which is lowered to a predetermined level or below is detected by the vacuum valve controller 311, the vacuum valve controller 311 switches the negative pressure supplied to the vacuum valve 307 to the atmospheric pressure, thereby closing the vacuum valve 307 to stop drawing the sewage in through the suction pipe 303.
Since the vacuum valve controller 311 opens and closes the vacuum valve 307 by utilizing the vacuum pressure of the vacuum sewage pipe 305, the opening time of the vacuum valve 307 depends on the degree of vacuum that is achieved in the vacuum sewage pipe 305 connected to the sewage pit 300. If the degree of vacuum in the vacuum sewage pipe 305 is low, then the opening time of the vacuum valve 307 is short, and hence the vacuum valve 307 may be closed after sucking in only sewage without sucking in air. As a result, water hammer is generated in the vacuum valve 307 to cause the vacuum valve 307 to drop out of the suction pipe 303, or to cause damage to the vacuum valve 307. Further, because air needed for delivering sewage does not flow in, an air lock tends to be developed in the pipe system,
The air lock refers to a phenomenon in which sewage is accumulated at an upstream side of a lift (a short upgrade step provided to reduce a burial depth of the vacuum sewage pipe 305 after the vacuum sewage pipe 305 is laid linearly on a downgrade in the ground), and there is no vent hole portion. If the air lock is developed, then the degree of vacuum required to deliver sewage is not achieved at the end of the vacuum sewage pipe 305, making it difficult to deliver sewage.
A vacuum valve controller for solving the above problems is disclosed, for example, in Japanese patent application No. H8-244194 (Japanese laid-open patent publication No. 10-60995). The disclosed vacuum valve controller has two nozzles mounted on upstream and downstream points of the suction pipe 303 for detecting respective pressures in those upstream and downstream points. Based on the difference between the detected pressures, it is determined whether sewage is flowing through the suction pipe 303 or air is being drawn through the suction pipe 303. When the pressure of air in the pressure sensor 309 increases to a predetermined value, the vacuum valve controller 311 opens the vacuum valve 307 to suck sewage from the suction pipe 303. Thereafter, the vacuum valve controller 311 closes the vacuum valve 307 when there is no sewage in the suction pipe 303 and the start of drawing in air is detected. Since the pressure difference is constant regardless of the degree of vacuum that is achieved, the vacuum valve 307 is always closed after the vacuum valve 307 sucks in air. Therefore, no water hammer occurs, and an air lock is hardly developed in the pipe system.
However, if the existing vacuum valve controller 311 shown in FIG. 1 is to be replaced with the vacuum valve controller disclosed in Japanese patent application No. H8-244194 (Japanese laid-open patent publication No. 10-60995) for the purpose of solving the air lock or the like, then not only the vacuum valve controller 311 needs to be replaced, but also two nozzles are required to be installed on the upstream and downstream points of the suction pipe 303 for detecting respective pressures in those upstream and downstream points. This modification is highly laborsome and time-consuming. The suction pipe 303 may be replaced with a suction pipe combined with two nozzles. However, because such a suction pipe combined with two nozzles has a cross-sectional shape different from that of the existing suction pipe 303, a partition plate 316 which divides the sewage tank 301 and the area where the vacuum valve 307 and its associated components are installed from each other needs to be modified. Therefore, a large replacement expense is required.